Xerographic charging apparatus with adjustable means to terminate the charging cycle when a predetermined charge is obtained



Aug. 8, 1967 p F N 3,335,275

XEROGRAPHIC CHARGING APPARATUS WITH ADJUSTABLE MEANS TO TERMINATE THE CHARGING CYCLE WHEN A PREDETERMINED CHARGE IS OBTAINED Filed Dec. 4, 1964 2 Sheets-Sheet 1 FIG. 1

mvezvron PAUL F. KING BY 76/5 2 ATTORNEYS Aug. 8, 1967 KlN 3,335,275

XEROGRAFHIC CHARGING APPARATUS WITH ADJUSTABLE MEANS TO TERMINATE THE CHARGING CYCLE WHEN A PREDETERMINED CHARGE IS OBTAINED Filed Dec. 4, 1964 2 Sheets-Sheet 2 rgr imam 58 v 50 HIGH VOLTAGE n D. C. POWER SUPPLY D. C. POWER SUPPLIES INVENTOR PAUL F. KING BY%,,/4ZZ

ATTORNL'VS United States Patent ()fifice 3,335,275 Patented Aug. 8, 1967 3,335,275 XEROGRAPHIC CHARGING APPARATUS WITH ADJUSTABLE MEANS T TERMINATE THE CHARGING CYCLE WHEN A PREDETERMINED CHARGE IS OBTAINED 5 Paul F. King, Webster, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Dec. 4, 1964, Ser. No. 415,965

3 Claims. (Cl. 250-495) 1 ABSTRACT OF THE DISCLOSURE A circuit for charging an electrostatic plate having a capacitor connected in series with the plate and a circuit arrangement for controlling a charging current in accordance with a predetermined charge developed across the capacitor.

This invention relates to the field of xerography and,

particularly, to an improved electric circuit to control a corona generating device for applying electrostatic charge on an in-place xerographic plate.

By present techniques, the charging of an in-place xerographic plate in preparation for the exposure step is accomplished by means of a stationary corona generating device whereby an electrostatic charge on the order of 600 volts is applied to the xerographic plate. A form of corona generating device for this purpose may comprise a plurality of parallel wires connected in series to a high Voltage source and supported in a conductive shield that is arranged in closely spaced relation to the surface to be charged. When the wires are energized, corona is generated along the surface of the wires and ions are caused U to be deposited on the adjacent photoconductive surface. A biased wire shield placed between the corona wires and the xerographic plate permits energizing the corona wires to a potential well above the corona threshold potential thereof without causing damage to the xerographic plate because the excess of corona current over that required for proper charging of the plate is drained off by the biased shield.

In the practice of in-place charging it has been found that equal uniformity of charging was not always produced. In particular, it was determined that the plate was charged in a pattern corresponding to the pattern of the corona discharge wires in the charging device. By closely spacing the multiplicity of corona wires the pattern effect on the plate is still produced except that the pattern will reflect the closeness of the corona wires.

As is well known, the corona threshold potential and the corona current from an energized wire are functions of the corona current for any given potential decreases as the wire diameter is increased. Variations in the potential applied to corona wires of a given diameter will cause relatively large changes in corona current with corresponding variations in the charging rate. In addition, the corona threshold potential and corona current are also affected directly by deposits of dust that may accumulate on the Wire, by atmospheric conditions such as humidity, temperature and pressure, and by variations of the ionized conditions of the air sheath surrounding the wire. Thus when operating at the corona threshold, minute differences in wire diameter, slight accumulations of dust on the wire, and variations in air current, atmospheric conditions and the spacing between the wire and the xerographic plate drastically affect the corona generating potential of the wire and cause a non-uniform electrostatic charge to be deposited on the xerographic plate.

Heretofore, it has been the practice for charging an in-place xerographic plate to adjust the corona voltage or the time which the corona wires are energized to give the proper voltage on the plate in order to correct for day-to-day changes in atmospheric conditions, changes in the altitude of the charging device, changes in the power line voltage, selenium coating and the aging of circuit components.

In the art of xerography it has been established that consistent high quality reproductions can best be effected when a correct and uniform potential is applied to a xerographic plate to prepare the plate for the exposure step. If the xerographic plate is not charged to a sufiicient potential, the elastrostatic latent image obtained upon exposure will be relatively weak and the resulting deposition of a developer material thereon will be correspondingly small and if the xerographic plate is overcharged, the converse will occur, and, if overcharged sufficiently, the photoconductive layer of the xerographic plate may be permanently damaged.

Since the contrast value, comparable to the contrast values obtainable from silver halide papers, of the electrostatic latent image is related directly to the potential charge on the xerographic plate before exposure, it is apparent that if the plate is not uniformly charged over its entire area, the contrast value of the electrostatic latent image obtained upon exposure will vary in different areas on the plate, and a streaky effect will be visible on the image when developed.

It is, therefore, the principal object 'of this invention to improve the electrical circuit of a corona generating device whereby a correct and uniform electrostatic charge may be deposited on a xerographic plate.

Another object of the present invention is to improve a corona charging device for the in-place charging of a xerographic plate that will insure complete and uniform charging of the entire plate and thereby eliminate striations, charge patterns or overcharging of the plate.

Still another object of the invention is to adapt a corona discharge device and circuit therefor that is particularly useful to charge a xerographic plate uniformly in a compact xerographic apparatus.

A further object of this invention is to improve a corona generating device for use in xerographic apparatus wherein it is desirable to charge an in-place xerographic plate to a uniform potential regardless of variations in the supply line voltage, or changes in the surrounding atmospheric conditions, structural variations of the control circuit element, or unevenness in a xerographic plate.

These and other objects of the invention are attained by cooperative action with a corona generating device comprising a mounting frame, coronode wires mounted and extending within the frame to charge a xerographic plate by corona discharge, and a charging control circuit. The charging circuit for ensuring a constant charging current includes a capacitor connected in series with a xerographic plate and a circuit arrangement for controlling the charging current in accordance with a predetermined charge developed across the capacitor.

For a better understanding of the invention as well as other objects and features thereof, reference is had to the following detailed description of the invention to be read in connection with the accompanying drawings, wherein:

FIG. 1 illustrates schematically an arrangement of a xerographic plate and a corona generating device in accordance with the invention;

FIG. 2 is a cross-sectional view of the corona charging device taken along the line 11-11 of FIG. 1; and,

FIG. 3 is a schematic electrical Wiring diagram of the control circuit for the corona generating device of FIG. 1.

Referring now to the drawings, there is disclosed a preferred arrangement of a corona generating device of the invention as applied to a xerographic plate. As shown in FIGS. 1 and 2, the present invention is adapted to be applied to a zerographic plate layer or radiation-receiving surface 10, on a conductive backing 12.

For the purpose of the present disclosure, the several xerographic processing stations for practicing zerography may be described functionally, as follows:

A charging station, at which a uniform electrostatic charge is deposited on the photoconductive layer of the Xerographic plate;

An exposure station, at which an optical image of a document to be reproduced is projected on the photoconductive layer to dissipate the plate charge in the exposed areas thereof and thereby form a latent electrostatic image of the document to be reproduced;

A developing station, at which a xerographic developing material including toner particles having an electrostatic charge opposite to that of the electrostatic latent image is cascaded over the plate surface, whereby the toner particles adhere to the electrostatic latent image to form a xerographic powder image in the configuration of the document to be reproduced;

A transfer station, at which the xerographic powder image is electrostatically transferred from the plate surface to a transfer material or support surface; and

A fixing station whereat the powder image on the support surface isv fused or permanently fixed to the surface.

The present invention is concerned with the charging function in a zerographic apparatus and contemplates that type of apparatus which has means for moving either the xerographic plate or the corona generating device to the charging station where charging is performed while these two structures are fixed relative to one another.

Referring now to the subject matter of the invention,

having a photoconductive such as selenium,

the electrostatic charging of the xerographic plate 10, 12

is accomplished by means of a corona generating device whereby an electrostatic charge is applied to the xerographic layer as it is positioned adjacent the charging device. The charging device comprises a flat frame structure 14 having side members 16, 18 spaced apart by transverse members 20, 22 which connect the ends of the side members, respectively, to form a frame structure with a central opening 24.

Preferably, the frame 14 is made of an insulating material, such as wood or plastic, and is formed with a first shoulder 26 upon which is mounted a back-up plate or shield 28 electrically grounded by any suitable means. The shield 28, which is composed of electrically conductive material such as metal, conductively coated glass or the like, serves as a terminal for the ions which are traveling in that direction and, also, to increase the electric field around the corona wires.

Another shoulder 30 is formed on the frame and the internal edges thereof define the sides of the opening 24 for the frame. Upon this shoulder are mounted two wires 32, 34 which are strung along in a criss-cross, overlapping fashion across the opening 24. The wire 32, connected at one end to a terminal 36 mounted on the side of the frame14, is stretched betweenthe side members 16, 18,

extends across the opening 24 as a series of fine corona discharge wires, and terminates at a fixed post 38. Similarly, the wire 34, connected at one end to a terminal 40, is stretched between the side members 16, 18, extends 1 across the opening 24 as a series of fine corona discharge wires, and terminates at a fixed post 42. The arrangement of the wires 32, 34 is such that there extends across the opening 24 a plurality of parallel corona wires which are evenly spaced, in a common plane, and subtend parallel with the plane of the photoconductive layer 10.

The Wires 32, 34 are made of any suitable non-corrosive material, such as stainless steel, having a uniform exterior. In a preferred embodiment of the charging device, the diameter of the wires is approximately three thousandths of an inch, it being apparent that other size wire can be used. Across the opening 24 and mounted on the frame 14 adjacent the plate 10, 12 is a metallic mask 44 ing 24. The mask is insulated from the xerographic plate and is centered over the photocondu-ctive surface 10.

The corona charging circuit of FIG. 3 is provided with a DC. power source in the form of a rectifier circuit generally indicated by the reference numeral 50. The rectifier circuit is connected to a pair of conductors 52, 53 for a source of alternating current, such as a commercial outlet of 120 volts and is adapted to produce a DC. potential in the range of approximately 2,000 to 8,000 volts. The output of the power source 50 is connected by a conductor 54 to the center tap of the secondary 56 of a high voltage transformer 58 which has its primary winding 60 connected to the supply conductors 52, 53. One end of the secondary 56 is connected to the terminal 36 and the other end is connected to the terminal 40 for supplying the wires 32, 34 with high AC. voltage. A pair of resistors 62, 63 are interposedin these connectors and serve as current stabilizers and aid to arrest air breakdown between the various wires 32, 34. With this arrangement there is a DC. and AG. potential supplied to the corona wires 32, 34 for sensitizing or charging the xerographic plate 10, 12.

Connected in series in the supply conductor 52 is a com trol circuit, generally indicated at 64, which serves to control theflow of electrical power to the DC. source 50 i normally open switch 70 of a charging relay 72. The

switches 68 and 70 are interposed in the source conductor 52 and thereby are adapted to control energization of the corona wires 32, 34. Selective actuation of the switch 70 isperformed by an operator by means of a start switch 74 connected in series with the relay 72 and the source conductors 52, 53.

Energization and control of the relay 66 is performed by the pentode section 76 of a 6U8 thermionic tube wherein the coil for the relay is in the plate circuit for the pentode. The relay is also connected to a source 78 of DC potential which also serves as the plate supply for the tube. When the tube is biased below cutoff and not conducting, the current in the plate circuit is insufficient to maintain energization of the relay 66 resulting in the continuance of the normally closed condition of the switch 68. Conduction of the pentode section 76 is under the control of the amount of charge on a capacitor 80 connected between the Xerographic plate 10, 12

and ground. At a point between the capacitor and the I xerographic plate, a conductor 82 is connected through a resistor 84 to the grid 86 of the pentode 76. The cathode 88 for the tube is connected to a ground conductor 90 through a capacitor 92 and is also connected to the cathode 94 of the triode section 96 of the 6U8 tube. The cathode 94 is also connected to the ground, conductor throughv a resistor 98 and the plate supply for the triode is derived from the DC. source 78 in the conventional manner.

The side of the capacitor 80 that is connected to the xerographic plateis also connected by a conductor 100 through a shunt resistor 102 to one contact of a normally closed switch 104 which is part of the relay 72. Upon energization of this relay by the operator in order to start charging of the xerographic plate, the switch 104 is actuated to its open condition. The other contact of the switch 104 is connected to the conductor 90 for electrically grounding the capacitor when a charging cycle has been terminated with the opening of the starting switch 74..

The operating potential of the pentode cathode 88 is controlled by the circuit for the triode 96 which includes a potentiometer 106 and battery 108 with a Wiper arm 110 for the potentiometer being connected to the control grid of the triode. Control is effected by the connection of the triode cathode 94 with the cathode 88 of the pentode. With this arrangement, the triode circuit may be adjusted by means of the potentiometer for establishing a predetermined threshold potential above which the pentode will conduct and thereby control conduction Of the pentode. As previously stated, conduction of the pentode will produce energization of the relay 66 for terminating charging to the corona wires 32, 34.

The potentiometer 106 may be varied for presetting the pentode cathode potential at which conduction of the pentode will be taking place for any particular grid voltage impressed upon the control gride 86 for the pentode. The value of this grid voltage will be determined by the amount of charge on the capacitor 80.

In operation, the present invention takes advantage of the Xerographic plate 10, 12 capacitance and its relationship with the capacitance of capacitor 80. The capacitor 80, by being connected in series with the Xerographic plate, becomes charged along with the plate during the charging cycle and the charging rate for the capacitor will reflect the relationship between the capacitances of the plate and the capacitor. The capacitance for the capacitor should be approximately 30 times larger than the plate capacitance so that a desired charge of 600 volts on the plate will develop a potential of 20 volts on the capacitor. This potential of 20 volts will be impressed upon the grid of the pentode 76 and be matched against the potential on the cathode 88 as determined by the potentiometer 106 and the triode circuit. Assuming the potential on the cathode is 22 volts and that of the grid 86 is 20 volts, the cathode-grid potential for the pentode will be 2 volts which may be set as sufficient grid voltage to give a plate current that will energize the relay 66 sufliciently to open the switch 68.

To start the corona generating device for charging the Xerographic plate 10, 12, the operator closes the start switch 74. This energizes the relay 72 which causes closing of the relay switch 70 for connecting the DC. power supply 50 and the AC. supply (transformer 58) to the source, represented by the source conductors 52, 53. The DC. power supplies 78 are directly connected to the conductors 52, 53 and may be energized separately from the control circuit 64. Closing of the relay switch 70 connects the DC. power supplies to the circuit 64. With this action, the corona wires 32, 34 become energized to produce a charge on the Xerographic plate. Simultaneously with the closing of the switch 70, the energized relay 72 also opens the switch 104 which disconnects the resistor 102 from the control circuit 64 during the charging cycle and allows the capacitor 80 to become charged during charging of the xerographic plate.

The xerographic plate is charged at a constant rate by the deposit of ions on its surface and the potential of the capacitor 80 will rise at a rate of that of the Xerographic plate. When the plate potential reaches 600 volts, the grid cathode potential of the pentode 76 will reach 2 volts causing energization of the relay 66. This action, in turn, will open the switch 68 for opening the DC. and AC. circuits to the corona wires and the plate charging will cease. After the charging has terminated, the start switch 74 is once again manually opened thus causing opening of the switch 70, deenergization of the relay 66, closing of the switch 104 for shorting out or discharging the capacitor, ground of the plate 12 through the resistor 102 and the dissipation of the bias on the grid 86. This will condition the pentode 76, the relay 66 and the capacitor for another charging cycle.

In attaining a plate potential of 600 under control of the circuit 64, the charging time is automatically adjusted for changes in supply voltages, atmospheric conditions, and the altitude at which charging is being performed. In the event one of these conditions changes during the day or from day to day, the control circuit 64 will insure full charging of the Xerographic plate by automatically adjusting the time of charging. Changes in a desired charging potential may be made by varying the potential 6 on the grid of the triode 96 by moving the wiper arm of the potentiometer 106. The charging time for the control circuit 64 for the parameters indicated is approximately .35 second and this may be varied by varying the output of the supplies 50, 58 in order to achieve a 600 volt potential on the plate.

With the control circuit of the invention, a predetermined charge can be produced uniformly on a xerographic plate and maintained at this predetermined level regardless of the normal variables such as those enumerated in the preceding paragraph. By maintaining the charging current constant, high quality reproductions having the desired contrast value can be made continuously and automatically. Striations and charge patterns on the Xerographic plate are minimized by balancing the AC. and DC. voltages which energize the corona wires 32, 34. These effects are also minimized by the use of the resulting A.C. biased D.C. charging current placed upon the criss-cross, overlapping arrangement of the various sections of the corona wires.

While the invention has been described with reference to the circuit disclosed herein, it is not confined to the details set forth since it is apparent that certain electrical equivalent components may be substituted for the components of the preferred circuit without departing from the scope of the invention. This application is therefore intended to cover such modifications or changes as may come within the purpose of the invention as defined by the following claims.

What is claimed is:

1. An apparatus for charging a xerographic plate including a back-up plate and at least one corona discharge wire positioned in relation to a grounded Xerographic plate to impose an electrostatic charge onto the Xerographic plate; a source of charging current being coupled to said corona discharge wire for charging the same; a control tube having a cathode, an anode and a grid; a relay connected in series with said anode and being energizable during conduction of said tube; said relay including a normally closed relay switch interposed between said corona wire and said source and actuable to an open condition when said relay is energized; a capacitor electrically connected between the xerographic plate and said cathode and adapted to become charged during charging of the plate, said capacitor being coupled to the grid of said control tube for imposing a bias on said grid in accordance with the charging current through the Xeroraphic plate, means for adjusting the operating potential of said control tube for establishing the cutoff bias above which the tube conducts, said capacitor being adapted to increase the grid bias above its cutoff point when the Xerographic plate has been charged to a predetermined level to control conduction of said tube for energizing said relay thereby maintaining the charging current on the corona wire at said predetermined level.

2. An apparatus for charging a Xerog-raphic plate including a back-up plate and at least one corona discharge wire ositioned in relation to a grounded xerographic plate to impose an electrostatic charge onto the xerographic plate; a source of alternating current; a high voltage direct current supply coupled to said corona discharge wire for charging the same with direct current biased alternating current; a control tube having a cathode, an anode and a grid; a relay connected in series with said anode and being energizable during conduction of said tube; said relay including a normally closed relay switch interposed between said corona wire and said source and actuable to an open condition when said relay is energized; a capacitor electrically connected between the xerographic plate and said cathode and adapted to become charged during charging of the plate, said capacitor being coupled to the grid of said control tube for imposing a bias on said grid in accordance with the charging current through the Xerographic plate, means for adjusting the operating potential of said control tube for establishing the cutoff bias above which the tube conducts, said capacitor being adapted to increase the grid bias above its cutoff point when the xerographic plate has been charged to a predetermined level to control conduction of said tube for energizing said relay thereby maintaining the charging current on the corona Wire at said predetermined level.

3. An apparatus for charging a Xerographic plate including a back-up plate and at least one corona discharge wire positioned in relation to a grounded xerographic plate to impose an electrostatic charge onto the Xerographic plate; a source of alternating current; a high voltage direct current supply coupled to said source and being coupled to said corona discharge wire for charging the same with direct current biased alternating current; a first control tube having a cathode, an anode and a grid; a relay connected in series with said anode and being energizable during conduction of said tube; said relay including a normally closed relay switch interposed between said corona wire and said source and actuable to an open condition when said relay is energized; a capacitor electrically connected between the xerographic plate and said cathode and adapted to become charged during charging of the plate, said capacitor being coupled to the grid of said control tube for imposing a bias on said grid in accordance with the charging current through the Xerographic plate; a second control tube having an anode, a cathode coupled to the cathode of said first tube and a grid, means for establishing a predetermined voltage on the grid of said second tube for adjusting the operating potential of said first control tube and thereby establish the cutoff bias above which the first tube conducts, said capacitor being adapted to increase the grid bias above its cutoff point when the Xerographic plate has been charged to a predetermined level to control conduction of said first tube for energizing said relay thereby maintaining the charging current on the corona wire at said predetermined level.

References Cited UNITED STATES PATENTS 3,076,092 1/1963 Mott 25049.5

RALPH G. NILSON, Primary Examiner. W. F. LINDQUIST, Assistant Examiner. 

1. AN APPARATUS FOR CHARGING A XEROGRAPHIC PLATE INCLUDING A BACK-UP PLATE AND AT LEAST ONE CORONA DISCHARGE WIRE POSITIONED IN RELATION TO A GROUNDED XEROGRAPHIC PLATE TO IMPOSE AN ELECTROSTATIC CHARGE ONTO THE XEROGRAPHIC PLATE; A SOURCE OF CHARGING CURRENT BEING COUPLED TO SAID CORONA DISCHARGE WIRE FOR CHARGING THE SAME; A CONTROL TUBE HAVING A CATHODE, AN ANODE AND A GRID; A RELAY CONNECTED IN SERIES WITH SAID ANODE AND BEING ENERGIZABLE DURING CONDUCTION OF SAID TUBE; SAID RELAY INCLUDING A NORMALLY CLOSED RELAY SWITCH INTERPOSED BETWEEN SAID CORONA WIRE AND SAID SOURCE AND ACTUABLE TO AN OPEN CONDITION WHEN SAID RELAY IS ENERGIZED; A CAPACITOR ELECTRICALLY CONNECTED BETWEEN THE XEROGRAPHIC PLATE AND SAID CATHODE AND ADAPTED TO BECOME CHARGED DURING CHARGING OF THE PLATE, SAID CAPACITOR BEING COUPLED TO THE GRID OF SAID CONTROL TUBE FOR IMPOSING A BIAS ON SAID GRID IN ACCORDANCE WITH THE CHARGING CURRENT THROUGH THE XEROGRAPHIC PLATE, MEANS FOR ADJUSTING THE OPERATING POTENTIAL OF SAID CONTROL TUBE FOR ESTABLISHING THE CUTOFF BIAS ABOVE WHICH THE TUBE CONDUCTS, SAID CAPACITOR BEING ADAPTED TO INCREASE THE GRID BIAS ABOVE ITS CUTOFF POINT WHEN THE XEROGRAPHIC PLATE HAS BEEN CHARGED TO A PREDETERMINED LEVEL TO CONTROL CONDUCTION OF SAID TUBE FOR ENERGIZING SAID RELAY THEREBY MAINTAINING THE CHARGING CURRENT ON THE CORONA WIRE AT SAID PREDETERMINED LEVEL. 